Q&A:
(1)Question: Anyone know which of the two fuse slots is for the fuse you've tapped into and which is for the circut you're running off it?
Answer: Where the blue fuse is the spot for the additional component that you're adding. The other slot is for the original fuse. I've verified this with my multimeter. 
(2)Question: Does it void new car's warranty if you add a circuit fuse?
Answer: it will not void because you are not permanently modifying nor damaging the car. use the correct fuse and the fuse will prevent any electrical damage. I use these to have my Car DVR automatically power up when i turn my car on. And automatically shuts down car DVR when i turn the car off.
(3)Question: These provide only one lead, so if I want to wire up an accessory, where would I connect my negative lead?
Answer: To any body point such as a screw to body, or frame, even under the dash to a bracket bolted to the firewall. 
Selling point:
1.High quality copper material made for the connector 
2.Tinplating treatment for the surface of the connector, anti oxidation and corrosion resistance 
3.Solder silver processing at the seam crossing between the end of the terminal and the end of the joint, with rib line inside the hole to increase compressive resistance 
4.Apply Vinyl, PVC and nylon material for the insulation sleeve, working ultimate temperature is 75 degree and 105 degree respectively 
5.Insulation sheath enclosing copper ring is double pressure type end and connectors, which can maintain reliable performance under vibration severe environment 

Description
Features:
Top of the line with electro-tin plated copper cores to prevent corrosion & engineered to hold the strongest crimps
Durable female and male fully insulated nylon quick disconnects
A.W.G.: 16-14, Wire range: 1.5-2.5mm², Max current=15A
Easy and Convenient to Use:Only one step connection of wires, insulated and sealed.
Specification:
Connector material: Copper
Color: Blue
Width: 6.8mm
Thickness: 0.8mm
Terminal kind: female/male
Wire cross section: 1.5 -2.5mm2 16-14 AWG
Mounting: crimp, on cable
Package includes:
1 bag*Female Male Electrical Wiring Connector(other accessories demo in the picture is not included.)
Note:
Please allow 1-2mm errors due to manual measurement.
Due to the difference between different monitors, there are color differences, please take the real product as the standard.